401 Unauthorized only when I user Get followers/ids API

[elsharkawy_sara]

I am using LinqToTwitter to get all followers of some users. I face 401 unauthorized error, despite other API requests are working. Here is my code:

string consumerKey = ConfigurationManager.AppSettings["consumer_key"];
string consumerSecret = ConfigurationManager.AppSettings["consumer_secret"];
string tokenKey = ConfigurationManager.AppSettings["token"];
string tokenSecret = ConfigurationManager.AppSettings["token_secret"];
var auth = new SingleUserAuthorizer
 {
                Credentials = new SingleUserInMemoryCredentials
                {
                    ConsumerKey = consumerKey,
                    ConsumerSecret = consumerSecret,
                    TwitterAccessToken = tokenKey,
                    TwitterAccessTokenSecret = tokenSecret
                }
 };
var twitterCtx = new TwitterContext(auth);
var followerIDs =
                             (from follower in twitterCtx.SocialGraph
                              where follower.Type == SocialGraphType.Followers &&
                                    follower.UserID == ulong.Parse(userID)
                              select follower)
                             .SingleOrDefault();

 followerIDs.IDs.ForEach(id => store(id, userID, followers));